Fiber reinforced thermoplastic composites and methods of making

ABSTRACT

Methods of making a fiber-reinforced thermoplastic polyurethane composite are described. The methods may include applying a sizing composition to a plurality of fibers to make sized fibers, where the sizing composition may include at least one curative for a thermoplastic polyurethane prepolymer. The sized fibers may be contacted with a thermoplastic polyurethane prepolymer composition to form a resin-fiber amalgam, where the thermoplastic polyurethane prepolymer composition includes 50 wt. % or less of a total amount of the curative that is also present on the sized fibers. The resin-fiber amalgam may then be cured to form the fiber-reinforced thermoplastic polyurethane composite.

BACKGROUND OF THE INVENTION

Thermoplastic resins are commonly combined with fibers, particles, andother solids to make reinforced composites. The solids in the compositecan increase tensile strength, dimensional stability, and other physicaland mechanical characteristics of the articles made with the composites.For example, glass fibers may be combined with a thermoplastic resincomposition to produce a glass fiber-reinforced composite (GFRC) withhigh tensile strength and improved rigidity. The glass fibers may takevarious forms, such as continuous or chopped strands, rovings, woven ornon-woven fabrics, and continuous or chopped strand mats.

The thermoplastic resin composition and glass fibers may be supplied bya extruder to a compression- or injection-molding machine so that theycan be formed into the GFRC. Typically, resin pellets and fiberglass arefed together or separately into the extruder. During the extrusionprocess using single or twin-screw machines, the resin is melted and thefibers are dispersed throughout the molten thermoplastic resin to form afiber/resin mixture. Next, the fiber/resin mixture may be degassed,cooled, and formed into pellets. The dry fiber strand/resin dispersionpellets may then be fed to a molding machine and formed into moldedcomposite articles that have a substantially homogeneous dispersion ofglass fibers throughout the composite article.

One difficulty with combining the thermoplastic resin composition andthe fibers is the typically high viscosity of the melted resincomposition. In many instances, thermoplastic polymers heated to amelting temperature have a viscosity well above room temperature water,more akin to a carbohydrate syrup such as room temperature molasses.Increasing the temperature of melted thermoplastic resin further canreduce the viscosity, but also increases the risk the resin willthermally decompose.

Thus, there is a need for developing methods of making fiber-reinforcedthermoplastic composites that permits a thermplastic resin compositionto be combined with fibers at lower viscosity. There is also a need formethods of reducing the viscosity of the thermoplastic resin compositionwithout heating a polymerized thermoplastic resin to its decompositiontemperature. The present application addresses this and other challengesin making reinforced thermplastic composites.

BRIEF SUMMARY OF THE INVENTION

Methods of making fiber-reinforced composites are described where a lowviscosity pre-polymerized thermoplastic resin is used to form aresin/fiber mixture and at least one curative for the pre-polymerizedthermoplastic resin is provided on sized fibers used in the composite.The curative on the fibers faciliate the polymerization of a lowviscosity pre-polymerized thermoplastic resin that forms the polymermatrix of the composite. Exemplary curatives may include polyfunctional(i.e., difunctional, trifunctional, etc.) compounds that form covalentbonds with the prepolymers and developing polymers of a thermoplasticpolyurethane resin composition. The present methods allow the partial(and in some instances complete) removal of a curative from thepre-polymerized thermoplastic resin compostion introduced to the sizedfibers during the making of the fiber-reinforced composite. Thereduction or removal of the curative (or curatives) from thethermoplastic resin composition provides it a longer pot life and/orhigher processing temperature.

Also described are fiber-reinforced composites that are made with sizedfibers that contain at least one curative. Exemplary sized fibers mayinclude glass fibers that are first mixed with a sizing composition thatinclude the curative. The sizing composition can leave the surfaces ofthe sized fibers with some or all of the curative needed to faciliatethe completion of the polymerization of a thermoplastic resincomposition that makes contact with the fibers. In some examples, morethan one type of curative may be provided on the sized fibers and at thesame time reduced or removed from the thermoplastic resin composition.

The removal of a curative from the thermoplastic resin compostion cansignificanity increase the pot life of the resin composition. While thecurative is desirably designed to faciliate the polymerization of thethermoplastic resin composition under raised-temperature curingconditions, it can often further polymerization even under more mildconditions when the resin composition is mixed with the fibers. If thepolymerization occurs to a great extent, the resin composition becomestoo viscous to mix properly with the fibers and often has to bediscarded. For the purposes of the present Application, the pot life ismeasured from the time all the components have been added to thethermplastic resin composition to the time the resin composition becomestoo viscous to properly mix with the fibers.

Embodiments of the present methods may include methods of making afiber-reinforced polyurethane composite by applying a sizing compositionto a plurality of fibers to make sized fibers, where the sizingcomposition may include at least one curative for a thermoplasticpolyurethane prepolymer. The sized fibers may be contacted with athermoplastic polyurethane prepolymer composition to form a resin-fiberamalgam, where the resin composition includes 50 wt. % or less of atotal amount of the curative that is also present on the sized fibers.The resin-fiber amalgam may then be cured to form the fiber-reinforcedpolyurethane composite.

Embodiments of the invention also include methods of extending ashelf-life of a thermoplastic polyurethane prepolymer composition usedto make a fiber-reinforced polyurethane composite. The methods mayinclude applying a sizing composition to a plurality of fibers to makesized fibers, where the sizing composition comprises at least onecurative for a thermoplastic polyurethane prepolymer. The methods mayfurther include contacting the sized fibers with the thermoplasticpolyurethane prepolymer composition to form a resin-fiber amalgam, andcuring the resin-fiber amalgam to form the fiber-reinforced composite.The thermoplastic polyurethane prepolymer composition has at least twicethe shelf-life of a resin mixture comprising the thermoplasticpolyurethane prepolymer composition mixed with all the curative.

Embodiments of the invention further include fiber-reinforcedpolyurethane composites made from sized fibers. The sized fibers aremade by contacting a plurality of fibers with a sizing composition thatincludes at least one curative for a thermoplastic polyurethaneprepolymer. The fiber-reinforced composites also include a thermoplasticmatrix formed from a thermoplastic polyurethane prepolymer compositionthat lacked at the at least one curative.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. The features and advantages ofthe invention may be realized and attained by means of theinstrumentalities, combinations, and methods described in thespecification.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings wherein like reference numerals are usedthroughout the several drawings to refer to similar components. In someinstances, a sublabel is associated with a reference numeral and followsa hyphen to denote one of multiple similar components. When reference ismade to a reference numeral without specification to an existingsublabel, it is intended to refer to all such multiple similarcomponents.

FIG. 1 shows selected steps in a method of making a fiber-reinforcedcomposite according to embodiments of the invention;

FIG. 2A shows a simplified reaction scheme for the formation of TPUprepolymers and their reaction with a curative to form a TPU polymer;

FIG. 2B shows a portion of a TPU polymer; and

FIG. 3 shows exemplary fiber-reinforced composites made with the presentthermoplastic resin compositions and sized fiber combinations.

DETAILED DESCRIPTION OF THE INVENTION

The present methods include combining thermoplastic resin compositionwith fibers sized with a sizing composition that includes at least onecurative for the thermoplastic resin composition. The placement of someor all of the curative on the sized fibers permits lower concentrationsof these components in the thermoplastic resin composition. The lowerconcentration, or in some instances the absence, of the curativecomponents in the thermoplastic resin composition increases itspot-life. The low curative concentrations may also permit lowering theresin composition's viscosity by heating it to higher temperatureswithout premature polymerization.

FIG. 1 shows selected steps in methods 100 of making a fiber-reinforcedcomposite according to present methods. The methods 100 may includeproviding a sizing composition 102 that has at least one curative forthe thermoplastic resin composition used in the fiber-reinforcedcomposite. The curative present in the sizing composition variesdepending on the thermoplastic resin. For example, when thethermoplastic resin composition includes a thermoplastic polyurethaneprepolymer, the curative is selected to help polymerize thethermoplastic polyurethane (TPU) prepolymer under polymerizationconditions. The curative reacts with reactive moieties on thepolyurethane prepolymers (e.g. isocyanate moieties) to become part ofthe final thermoplastic polymer structure of the fiber-reinforcedcomposite. The curative is polyfunctional and capible of bonding withmore than one reactive site of the polyurethane prepolymer. The bondingreactions between the curative and the polyurethane prepolymers can formblock copolymers alternating between shorter and more polar “hard”segments and longer, less ordered, and more non-polar “soft” segments.The differences in chemical polarity can cause a phase separation of thehard and soft segments, and also create electrostatic attraction betweenthe hard segments that may increase the elasticity of the polymer.

Exemplary curatives for a thermoplastic resin composition may includeamines, organic acids, organic anhydrides, alcohols, and thiols, amongother curatives. When the thermoplastic resin composition includes athermoplastic polyurethane prepolymer, exemplary thermoplasticpolyurethane prepolymer curatives may include diamines that form ureabonds with the isocyanate groups on the prepolymer, diols that formurethane bonds with the isocyanate groups on the prepolymer, and hydroxyamines that have both amine and hydroxy functional groups that bond withthe prepolymer. They may also include polyfunctional curatives such aspolyols (e.g., triols), polyamines (e.g., triamines), and trialkanolamines, among others, that may provide limited covalent crosslinking ofthe thermoplastic polyurethane in the fiber-reinforced composite. Moregenerally, they may include a compound having the formula(X)_(n)—R—(Y)_(m), wherein X and Y are independently a hydroxyl group,or an amine group; n and m are independently an integer ranging from 1to 3; and R is an organic group with one or more carbon atoms.

Specific exemplary diamines may include ethylene diamine,1,2-diaminopropane, 1,3-diaminopropane, and 2,5-diaminotoluene amongother diamines. Specific exemplary diols may include ethylene glycol,propylene glycol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol,cyclohexane dimethanol, and hydroquinone bis(hydroxyethyl) ether (HQEE),among other diols. Specific exemplary hydroxy amines may include ethanolamine, and 1-amino-2-propanol, among others.

Exemplary curatives for thermoplastic polyurethane prepolymers may alsoinclude modified amines that contain at least one amine group and atleast one other functional group capable of reacting with thethermoplastic polyurethane prepolymer, such as a hydroxyl group (—OH),carboxyl group (—COOH), amide group (—C(═O)NH—), and/or an anhydridegroup, among other groups. Exemplary curatives may also include modifiedalcohols that contain at least one hydroxyl group and at least one otherfunctional group capable of reacting with the thermoplastic polyurethaneprepolymer, such as an amine group, carboxyl group (—COOH), amide group(—C(═O)NH—), and/or an anhydride group, among other groups.

Exemplary polyfunctional curatives may have three or more functionalmoities capable of reacting with the thermoplastic polyurethaneprepolymer. They may include glycerol, sorbitol, and sucrose, amongothers.

The sizing composition may also include additional compounds beyond thecurative for the thermplastic resin composition. For example, the sizingcomposition may include a solvent (e.g., water, ethyl alcohol), couplingagents, film-forming agents, lubricants, and wetting agent, among othercompounds. The coupling agents may act as chemical linking agents bybonding to both the glass fiber and the plastic matrix. Exemplarycoupling agents may include silanes containing organosilane groups thatmay be coupling agents for glass fibers and organic polymers, and serveto bond the two materials in the composite article.

In some examples, the coupling agent is a reactive coupling agent thatmay include a functional group that can form a covalent linking bondwith the thermoplastic resin composite. For example, the coupling agentmay have an amino silane group or a hydroxyl silane group. The amine andhydroxyl functional groups on the silane coupling compound can react toform covalent urea and urethane linkages, respectively, with athermoplastic polyurethane prepolymer.

Film forming agents may provide a desired degree of bonding between thefibers in the fiber strands to avoid fuzzing and excess filamentationduring processing in fiber manufacturing operations and/or fibercomposite fabrication operations. Lubricants help protect the surface ofthe fibers from scratches and abrasions commonly caused byfiber-to-fiber contact and friction during processing. Wetting agentsfacilitate the spread of the sizing composition on the surface of thefibers, and may also facilitate the spread of the thermoplastic resincomposition across the surface of the sized fibers.

Returning to FIG. 1, the above-described sizing composition may beapplied to the fibers 104. The application of the sizing composition tothe fibers may be achieved by kiss-roll coating, spraying, dipping,contacting, and/or mixing the liquid sizing composition and the fibers.The wet sized fibers may be exposed to elevated temperature and/orturbulent flow conditions to accelerate their drying.

Exemplary fibers used in the present sizing applications may include oneor more types of fibers chosen from glass fibers, ceramic fibers, carbonfibers, metal fibers, and organic polymer fibers, among other kinds offibers. Exemplary glass fibers may include “E-glass”, “A-glass”,“C-glass”, “S-glass”, “ECR-glass” (corrosion resistant glass),“T-glass”, and fluorine and/or boron-free derivatives thereof. Exemplaryceramic fibers may include aluminum oxide, silicon carbide, siliconnitride, silicon carbide, and basalt fibers, among others. Exemplarycarbon fibers may include graphite, semi-crystalline carbon, and carbonnano tubes, among other types of carbon fibers. Exemplary metal fibersmay include aluminum, steel, and tungsten, among other types of metalfibers. Exemplary organic polymer fibers may include poly aramid fibers,polyester fibers, and polyamide fibers, among other types of organicpolymer fibers.

The fiber length may range from short-to-intermediate chopped fibers(e.g., about 0.5 inches or less in length) to long fibers (e.g., morethan about 0.5 inches in length), and to continuous fibers and rovings,among others. In addition to imparting reactive curing properties to thefibers, the sizing composition may enhance the fibers' physicalcharacteristics in a number of ways including increased hardness,increased mechanical strength, greater wettability, and increasedadhesion between the fibers and resin.

Once the fibers are sized, they may then be contacted with thethermoplastic resin composition to make a fiber-resin amalgam 106. Asnoted above, one exemplary thermoplastic resin composition is athermoplastic polyurethane (TPU) prepolymer. Exemplary TPU prepolymersmay include reactive isocyanate groups that can react with the curativeto form a thermoplastic polyurethane polymer. In the presentembodiments, the thermoplastic polyurethane polymer forms the polymermatrix of a fiber-reinforced composite.

Referring now to FIG. 2A, the thermoplastic polyurethane (TPU)prepolymers 206 may be formed from the reaction of a diisocyanatecompound 204 with a polyol compound 202. Exemplary diisocyanatecompounds may include alkyl diisocyanates such as hexamethylenediisocyanate, and aryl diisocyanates such as toluene diisocyanate andmethylene diphenyl diisocyanate, among others. Exemplary polyols mayinclude long-chain polyols such as polyether-type polyols andpolyester-type polyols. Exemplary polyether-type polyols may includepolyethylene glycol, polypropylene glycol, poly(tetramethylene ether)glycol, and tetrahydrofuran ethers, among others. Exemplarypolyester-type polyols may include polyol products from the reactionbetween adipic acid (AA) and diethylene glycol. FIG. 2A shows anexemplary curative 208 that includes a diol with two reactive hydroxylgroup (—OH) as functional groups. It should be appreciated that theexemplary curative 208 is one of many types of curatives described abovethat can be used in the present methods.

In the reaction schematic illustrated in FIG. 2A, the polyol 202 and thediisocyanate compound 204 react to form a TPU prepolymer 206, and thenthe TPU prepolymer 206 reacts with the curative 208 to form thethermoplastic polyurethane (TPU) polymer 210. The TPU polymer 210includes a hard segement made of adjacent diisocyanate groups that arecovalently linked by curative, and soft segments made of long chains inthe reacted polyol. It should be appreciated that the TPU polymer 210shown may be a portion of a larger TPU polymer.

FIG. 2B shows a larger portion of a TPU polymer 212, that includes afirst phase of physically bonded hard segments incorporated into asecond phase of soft segments. The physically bonded hard segmentsprovide a physical crosslinking that imparts an elastomeric quality tothe TPU polymer 212. Unlike the permanent chemical crosslinking found inelastomic polymers like thermoset rubber, the physical crosslinking ofthe hard segments is reversible with the application of sufficient heat.

The thermoplastic resin composition may include the at least onecurative that is added to the resin composition prior to contact withthe sized fibers. When the sized fibers include a curative for thethermoplastic resin composition, a lower concentration of curative isrequired in the resin composition to complete the polymerization. Forexample, sized fibers that include a curative may contact athermoplastic resin composition having 50 wt. % or less of the curativein the resin composition. For example the resin composition may have 50wt. % or less, 40 wt. % or less, 30 wt. % or less, 25 wt. % or less, 20wt. % or less, 15 wt. % or less, 10 wt. % or less, 5 wt. % or less,etc., of the total amount of curative while the remainer is supplied bythe sized fibers. In some embodiments, all the curative may be providedby the sized fibers and there is no curative present in the resincomposition.

Exemplary weight percentage distribution ratios for the curative in thethermoplastic resin composition and the sized fibers may be 50:50[resin:fibers], 40:60, 30:70, 20:80, 15:85, 10:90, 5:95, and 0:100.These exemplary weight percentage distribution ratios also includeranges, including 50:50 to 40:60, 40:60 to 30:70, 30:70 to 20:80, 20:80to 15:85, 15:85 to 10:90, 10:90 to 5:95, and 5:95 to 0:100. They mayalso be extended across two or more of the above-described ranges, forexample 50:50 to 30:70, 50:50 to 0:100, 40:60 to 10:90, and so forth.

In embodiments where two or more curatives are used, they may be reducedindependently or collectively in the thermoplastic resin composition bythe above-describe amounts and distribution ratios for a singlecurative. In further embodiments where two or more curatives are used,the combined amount of the curatives in the resin composition may be 50wt. % or less, 40 wt. % or less, 30 wt. % or less, 25 wt. % or less, 20wt. % or less, 15 wt. % or less, 10 wt. % or less, 5 wt. % or less,etc., of the total amount of the curatives, while the remainder issupplied by the sized fibers. For example, the combined amount of thecuratives in the resin composition may be 50 wt. %. As noted above, thecombined amounts of curatives may have the above-described weightpercentage distribution ratios for the example with a single curative.

The combination of the sized fibers and thermoplastic resin compositionto make the fiber-resin amalgam may be achieved by thermoplasticcomposite manufacturing techniques, including resin-injection molding(RIM), strucutral resin-injection molding (SRIM), resin transfer molding(RTM), vacuum infusion, wet lay-up processes, spray-up processes,filament winding processes, and pultrusion processes, among otherprocesses. In some embodiments, the fiber-resin amalgam may be formedinto sheet molding compounds (SMCs) and/or bulk molding compounds (BMCs)that can be used in compression molding techniques to produce thefiber-reinforced composite.

Exemplary thermoplastic composite manufacturing techniques may alsoinclude direct-long fiber thermoplastic (D-LFT) molding techniques.D-LFT molding is a technology where thermoplastic resin composition isdirectly compounded with long glass fibers and then molded in oneoperation. Different from a conventional extrusion process in whichchopped fibers are used, in a D-LFT process continuous roving strandsare fed into extruder. The advantage of D-LFT is the ability to producesignificantly longer glass fibers in the final composite materials. Incomparison to a standard LFT process based on long fiber pellets, theD-LFT process doesn't produce semi-finished material. When D-LFT is usedin compression or injection molding, a melted resin-fiber compositionmay be transferred into a molding tool located in a compression press ordirectly injected into the mold.

Exemplary thermoplastic composite manufacturing techniques may furtherinclude long fiber injection (LFI) molding techniques. In LFI techniquesfor thermoplastic polyurethane fiber-reinforced composites, choppedfiber and polyurethane components are combined and delivered to a moldcavity in a continuous process. The LFI system may include a mixing headwith fiber cutting units to chop glass fibers to a specified length. Insome embodiments, polyol and diisocyanate that make up the polyurethanemay also be delivered to the mixing head, while in others thethermoplastic polyurethane prepolymer may be delivered. In either case,the polyurethane composition is combined with the chopped glass fibersin the mixing head. The mixing head may be passed over a mold as thefiber-composition combination from the mixing head is poured into themold, which may be heated. When the addition of the fiber-compositioncombination is complete, the mold may be closed and pressure may beapplied. After a curing time has passed (e.g. about 2 to 10 minutes),the fiber-reinforced composite article may be removed from the mold.When the present combinations of thermoplastic polyurethane prepolymercompositions and curative-containing sized fibers are used, there islower viscosity and less polymerization of the prepolymer composition inthe mixing head, resulting in less frequent cleaning, maintenance, andother downtime for the LFI system.

Accompanying or following the formation of the fiber-resin amalgam, thethermoplastic resin composition may be polymerized to form thefiber-reinforced composite 108. The polymerization conditions mayinclude elevating the temperature of the fiber-resin amalgam above apolymerization-threshold temperature. The exemplary thermoplasticpolyurethane resin systems may have a polymerization-thresholdtemperature ranging from room temperature (e.g., about 20° C.) to about180° C. (e.g., 100° C. to 150° C.).

FIG. 3 shows an exemplary fiber-reinforced composite wind turbine blade302 formed by the present fiber-resin amalgams. The blade 302 is one ofmany types of articles that can be formed by the amalgams. Otherarticles may include vehicle parts (e.g., aircraft parts, automotiveparts, etc.), appliance parts, containers, etc.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Additionally, a number of well-known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent invention. Accordingly, the above description should not betaken as limiting the scope of the invention.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassed.The upper and lower limits of these smaller ranges may independently beincluded or excluded in the range, and each range where either, neitheror both limits are included in the smaller ranges is also encompassedwithin the invention, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsoincluded.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise. Thus, for example, reference to “a process” includes aplurality of such processes and reference to “the fiber” includesreference to one or more fibers and equivalents thereof known to thoseskilled in the art, and so forth.

Also, the words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps, acts, orgroups.

What is claimed is:
 1. A method of making a fiber-reinforcedpolyurethane composite, the method comprising: applying a sizingcomposition to a plurality of fibers to make sized fibers, wherein thesizing composition comprises at least one curative for a thermoplasticpolyurethane prepolymer; contacting the sized fibers with thethermoplastic polyurethane prepolymer composition to form a resin-fiberamalgam, wherein the thermoplastic polyurethane prepolymer compositionincludes 50 wt. % or less of a total amount of the at least one curativethat is also present on the sized fibers; and curing the resin-fiberamalgam to form the fiber-reinforced polyurethane composite.
 2. Themethod of claim 1, wherein the thermoplastic polyurethane prepolymercomposition comprises 0 wt. % of the curative that is present on thesized fibers.
 3. The method of claim 1, wherein the thermoplasticpolyurethane prepolymer composition is the reaction product of a polyoland a polyisocyanate compound.
 4. The method of claim 3, wherein thepolyol comprises a polyester polyol or a polyether polyol.
 5. The methodof claim 4, wherein the polyester polyol comprises a reaction product ofadipic acid and diethylene glycol.
 6. The method of claim 4, wherein thepolyether polyol comprises a polyalkylene glycol.
 7. The method of claim1, wherein the sizing composition comprises at least one thermoplasticpolyurethane prepolymer curative.
 8. The method of claim 7, wherein thethermoplastic polyurethane prepolymer curative comprises a compoundhaving the formula (X)_(n)—R—(Y)_(m), wherein X and Y are independentlya hydroxyl group or an amine group; n and m are independently an integerranging from 1 to 3; and R is an organic group with one or more carbonatoms.
 9. The method of claim 7, wherein the thermoplastic polyurethaneprepolymer curative comprises a diamine, a diol, or a hydroxy amine. 10.The method of claim 1, wherein the sizing composition further comprisesa coupling agent.
 11. The method of claim 10, wherein the coupling agentcomprises a silane coupling agent.
 12. The method of claim 1, whereinthe sized fibers are contacted with the thermoplastic polyurethaneprepolymer composition by a thermoplastic composite manufacturingprocess chosen from resin-injection molding (RIM), strucutralresin-injection molding (SRIM), resin transfer molding (RTM), vacuuminfusion, a wet lay-up process, a spray-up process, filament windingprocesses, a pultrusion process, a direct long fiber thermoplastic(D-LFT) process, and a long fiber injection (LFI) process.
 13. A methodof extending a shelf-life of a thermoplastic polyurethane prepolymercomposition used to make a fiber-reinforced polyurethane composite, themethod comprising: applying a sizing composition to a plurality offibers to make sized fibers, wherein the sizing composition comprises atleast one curative for the thermoplastic polyurethane prepolymercomposition; contacting the sized fibers with the thermoplasticpolyurethane prepolymer composition to form a resin-fiber amalgam,wherein the thermoplastic polyurethane prepolymer composition has atleast twice the shelf-life of a thermoplastic polyurethane prepolymercomposition having all the thermoplastic polyurethane prepolymercurative that is present on the sized fibers; and curing the resin-fiberamalgam to form the fiber-reinforced polyurethane composite.
 14. Themethod of claim 13, wherein the thermoplastic polyurethane prepolymercurative comprises a compound having the formula (X)_(n)—R—(Y)_(m),wherein X and Y are independently a hydroxyl group or an amine group; nand m are independently an integer ranging from 1 to 3; and R is anorganic group with one or more carbon atoms.
 15. The method of claim 13,wherein the thermoplastic polyurethane prepolymer curative comprisesdiamines, diols, and hydroxy amines.
 16. The method of claim 13, whereinthe sizing composition further comprises a coupling agent.
 17. Themethod of claim 16, wherein the coupling agent comprises a silanecoupling agent.
 18. The method of claim 13, wherein the sized fibers arecontacted with the thermoplastic polyurethane prepolymer composition bya thermoplastic composite manufacturing process chosen fromresin-injection molding (RIM), strucutral resin-injection molding(SRIM), resin transfer molding (RTM), vacuum infusion, a wet lay-upprocess, a spray-up process, filament winding processes, a pultrusionprocess, a direct long fiber thermoplastic (D-LFT) process, and a longfiber injection (LFI) process.
 19. A fiber-reinforced polyurethanecomposite comprising: sized fibers that have been formed from aplurality of fibers contacted with a sizing composition comprising atleast one curative; and a thermoplastic polyurethane matrix formed froma thermoplastic polyurethane prepolymer composition that lacked at theat least one curative.
 20. The fiber-reinforced polyurethane compositeof claim 19, wherein the thermoplastic polyurethane prepolymercomposition is the reaction product of a polyol and a polyisocyanatecompound.
 21. The fiber-reinforced polyurethane composite of claim 20,wherein the polyol comprises a polyester polyol or a polyether polyol.22. The fiber-reinforced polyurethane composite of claim 21, wherein thepolyester polyol comprises a reaction product of adipic acid anddiethylene glycol.
 23. The fiber-reinforced polyurethane composite ofclaim 21, wherein the polyether polyol comprises a polyalkylene glycol.24. The fiber-reinforced polyurethane composite of claim 19, wherein thesizing composition comprises at least one thermoplastic polyurethaneprepolymer curative.
 25. The fiber-reinforced polyurethane composite ofclaim 24, wherein the thermoplastic polyurethane prepolymer curativecomprises a compound having the formula (X)_(n)—R—(Y)_(m), wherein X andY are independently a hydroxyl group or an amine group; n and m areindependently an integer ranging from 1 to 3; and R is an organic groupwith one or more carbon atoms.
 26. The fiber-reinforced polyurethanecomposite of claim 24, wherein the thermoplastic polyurethane prepolymercurative comprises diamines, diols, or hydroxy amines.
 27. Thefiber-reinforced polyurethane composite of claim 19, wherein the sizingcomposition further comprises a coupling agent.
 28. The fiber-reinforcedpolyurethane composite of claim 27, wherein the coupling agent comprisesa silane coupling agent.